Wash and peel the potatoes, put them into well-salted, boiling water with the garlic and cook until the potatoes are just tender (it is very important not to under- or overcook them). Drain them and let them sit for about 5 minutes to enable the excess water to evaporate (if you overcook them or don't drain them correctly they'll be too moist and your filling will be too wet).
When the potatoes have cooled slightly add your butter and your chosen cheeses. Stir and mash with a fork to mix and break the potatoes up (I like to leave the mixture slightly chunky). Add the nutmeg and seasoning to taste. Stir in the watercress, half finely chopped and the rest coarsely chopped. Stuff the ravioli with a good heaped teaspoon of this mixture and cook in boiling salted water for about 3-4 minutes, until tender.
Serve with some extra olive oil or butter, some more of your chosen cheese grated over the top and some torn watercress.